During flight, a boundary layer of low-energy air builds up on surfaces of an aircraft. The boundary layer is a thin film of low velocity, low dynamic pressure air located near a solid boundary resulting from the air being at rest along the solid boundary. The boundary layer which forms on surfaces located upstream of an engine can become ingested by the engine and decrease the recovery of total pressure and corresponding thrust performance. Further, the ingested boundary layer degrades distortion (i.e., a measurement of the quality or uniformity of flow characteristics) and thereby decreases stability of engine operation.
These problems are aggravated for inlets which require the flow to undergo any significant turn in flow direction. A turn produces large pressure gradients and secondary flows which increase thickness of the boundary layer or cause it to separate from the surface.
The boundary layer is especially susceptible to separate from a surface when a shock wave interacts with the boundary layer. That produces large viscous losses, low total pressure, and poor flow quality at the engine face.
Consequently, many aircraft have employed some type of boundary layer removal or reduction system to ensure stable engine operation and high aerodynamic performance. Example systems include boundary layer diverters, “bump” boundary layer deflectors, vortex generators, porous surfaces to “bleed” the flow, inlet throat slots, side wall cut back, air injection, and overboard bypass ducts. Unfortunately, these systems are complex and can entail a substantial increase in aircraft weight and/or volume. Moreover, they are optimized for one flight condition or throttle setting and are not adjustable. They can become less effective at a variety of speeds, altitudes, angles of attack or side-slip, and engine air flow rates.
The foregoing examples of related art and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.